Fault Bumps: Prevalence, Causes and Consequences 
Emily Brodsky 
UC Santa Cruz

Earthquake dynamics are strongly affected by fault zone structure and fault
surface geometry. Corrugations have been long-noted on fault surfaces. Recent
LiDAR measurements suggest that the along- slip wavelength of these
corrugations are ~10-40 m for mature faults. A detailed study of the interior
fault architecture and the fault topography of a fault zone near Klamath Falls,
Oregon combined with LiDAR measurements of the fault surface shows that the
bumps (asperities) are linked to the fault zone damage architecture. We find
that the fault zone has layered damage architecture, as is typical of exhumed
structures. Slip primarily occurs inside a 1-20 mm wide band that contains
principal slip surfaces with individual widths of ~100 mm. The slip band sits
atop a cohesive layer which deforms by granular flow. The bumps reflect
variations of the thickness of the granular cohesive layer and can be generated
by a boudinage-like instability. As the granular layer is stiffer than its
surroundings, the asperities are both geometrical and rheological
inhomogenities. Modeling slip along wavy faults shows that slip on a surface
with a realistic geometry requires internal yielding of the host rock.
Consistently, our observations suggest that deformation processes in the fault
zone include on-going fracturing, slip along secondary faults and particle
rotation. Slip surfaces localize on the border of the granular cohesive layer.
The on-going slip smoothes the surfaces and thus the structural and geometrical
evolution of the granular layer create a preference for the continuation of
slip on the same surface. There is a feedback cycle between slip on the surface
and the generation of the granular layer that then deforms and controls the
locus of future slip.